1. Field of the Invention
This invention relates to the heating of fluids such as liquids by means of solar energy. It has particular but not exclusive application to solar water heaters.
2. Description of the Prior Art
The use of solar energy on a large scale is now technologically practicable for industrial process heating and mineral and food processing plants where water at temperatures up to 100.degree. C. is required. Its more widespread use is inhibited by relatively low cost of fossil fuels and the convenience of natural gas and oil as a primary energy source. The impending energy crisis is likely to change this situation but even at present prices there are many climates throughout the world where solar energy can now compete where skillfully applied. For homes, hotels and medium temperature industrial applications requiring water at up to 55.degree. C. there are many successful solar water heating installations and in Australia there is a small but viable water heating industry.
The standard domestic solar water heater in Australia is used both with and without electric or fuel boosting, depending on climatic conditions. The collectors are manufactured typically in units 1220 mm by 610 mm (0.75 m.sup.2 in area), and incorporate a selective absorbing surface with a single glass cover. A selective absorbing surface is one which has a higher absorptance than emittance at the radiation wavelength concerned. Such surfaces may be produced by special black-oxide coatings and may typically have an absorptance of 0.8 but an emittance of 0.1 to 0.15 at wavelengths of 0.2 to 2 microns. Such heaters are typically used to heat water from 15.degree. C. and losses are about 75 Wm.sup.-.sup.2. At higher water temperatures the losses are normally twice this figure. By using more insulation and double glazing these losses could be reduced but the collector would then cost more and would be less efficient at low temperatures because of the reduction in insolation reaching the absorber plate of the collector. Since low temperature collectors are unsatisfactory at high temperatures and high temperature collectors are both more expensive and less efficient at low temperatures, there will be an optimum operating temperature range for each design of collector.